The invention relates to cutter assemblies for rotary drill bits for use in drilling or boring deep holes in subsurface formations.
The cutting assemblies are for use in rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a channel in the bit body for supplying drilling fluid to the surface of the bit for cleaning and/or cooling the cutters. Each cutter assembly comprises a generally cylindrical stud which is received in a socket in the surface of the bit body, the stud being formed, adjacent one end thereof, with a plane surface disposed at an angle of less than 90.degree. to the longitudinal axis of the stud, and a preform cutting element being mounted on the inclined surface of the stud. The preform cutting element may be of the kind comprising a tablet, usually circular, having a thin hard cutting layer of polycrystalline diamond bonded to a thicker, less hard backing layer, for example of tungsten carbide.
In order to provide adequate support for the stud in the bit body, and to locate the cutting edge of the cutting element in the desired relation to the surface of the bit body, it is normally desirable for a portion of the cutting element to lie below the surface of the bit body. In known forms of cutter assembly of the above-mentioned type, it has been usual for peripheral portions of the cutting element, as viewed axially of the stud, to project beyond the peripheral surface of the stud. This is normally the case even if the stud is of greater diameter than the cutting element, since the location of the cutting element on the inclined plane is such that its maximum width does not lie on the longitudinal axis of the stud, but lies to one side of that axis, where the width of the stud is less.
In order to allow a part of the cutting element to project below the surface of the bit body, therefore, it has normally been the practice to counterbore the mouth of the socket for receiving the stud so as to provide a clearance for the projecting portion of the cutting element. A disadvantage of this arrangement, however, is that the provision of the counterbore to clear the cutting element also removes some of the support which would otherwise be provided for the back of the stud by the bit body. In order to overcome this disadvantage, it has been proposed, as described in U.S. Pat. No. 4,465,148, that the counterbore should be eccentric with relation to the stud-receiving socket, the internal surface of one side of the counterbore forming a continuation of the internal surface of the socket so as to provide a continuous support for the back of the stud while at the same time providing clearance for the cutting element on the forward side of the stud.
However, such arrangement has the disadvantage of making the manufacture of the drill bit more complicated since the counterbore must be exactly positioned according to the required orientation of the cutter assembly. The present invention sets out to provide an improved cutter assembly, and method of manufacturing a drill bit, which provides the advantages of the eccentric counterbore arrangement while at the same time simplifying, rather than complicating, the manufacture of the drill bit, and thus reducing its cost.